The human gastrointestinal tract houses large and complex communities of microorganisms that are important for normal health, but can also contribute to diseases such as gastrointestinal cancer. In this proposal we will use a zebrafish model to investigate how factors produced by gut microbes interact with host genetic factors to drive the progression of intestinal cancer. We will make use of zebrafish lines that are predisposed to intestinal cancer due to over-activation of the Wnt signaling pathway, chronic inflammation, or loss of DNA repair mechanisms, all of which are commonly associated with gastrointestinal cancers in humans. In our first aim we will characterize a number of secreted proteins, produced by both human and zebrafish-associated bacteria, that modulate host programs associated with gastrointestinal cancer development, including epithelial cell proliferation and inflammation. We will determine whether the pro-proliferative proteins interacts synergistically with cancer-prone genetic backgrounds to disrupt normal programs of intestinal epithelial renewal and cause hyperplasia, and furthermore whether the anti-inflammatory proteins confer protection against hyperplasia. In our second aim we will characterize a transgenic zebrafish line that expresses the bacterial oncogene, CagA, from the stomach cancer-associated pathogen Helicobacter pylori. We will determine the host genetic programs that are activated during the progression of these transgenic fish to hyperplasia and cancer, and we will test which of these pathways are required for CagA's carcinogenic properties. We will also test whether CagA expression alters the intestinal microbial community in ways that promote intestinal pathology. In our third aim we will investigate the microbial etiology behind an outbreak of spontaneous intestinal cancers in our zebrafish facility, which we have shown to be transmissible. We will use the experimental framework of transmissible disease to identify microbes associated with intestinal pathology. We will employ our cancer- prone zebrafish lines to characterize the carcinogenic potential of disease-associated microbial communities and of individual candidate microbes. This research will provide new insight into the mechanisms by with intestinal microbes contribute to cancer development and will ultimately lead to new approaches to diagnose, treat and prevent gastrointestinal cancers in humans.